The present invention relates to an air-fuel ratio control system for an internal combustion engine emission control system with a three-way catalytic converter, and more particularly to a system for a vehicle for controlling the air-fuel ratio to the stoichiometric air-fuel ratio so as to effectively operate the three-way catalyst.
The control system comprises a feedback control system, in which an O.sub.2 -sensor is provided on the exhaust passage of the internal combustion engine. The O.sub.2 -sensor is adapted to sense the oxygen content of exhaust gases to generate an electrical output dependent on the oxygen content as a representation of the air-fuel ratio of the air-fuel mixture which is supplied to the engine cylinder by the carburetor of the engine. An electronic control circuit operates to judge whether the output of the O.sub.2 -sensor is higher or lower than a standard value corresponding to the stoichiometric air-fuel ratio and produces an output signal. The output signal is converted to a driving pulse train through a pulse generator and driving circuit, which is fed to an actuator for the carburetor. Thus, the air-fuel ratio of the mixture is controlled to the stoichiometric air-fuel ratio.
In the case that the O.sub.2 -sensor fails to detect the oxygen content or the feedback control circuit malfunctions, the air-fuel ratio of the mixture diverges to the rich or lean side from the stoichiometry value. To avoid such erroneous air-fuel ratios, a conventional control circuit is so arranged that when the extremely rich or lean air-fuel ratio continues for a predetermined period, feedback operation of the circuit is cut out and a constant output signal a providing medium air-fuel ratio is produced.
However, such a control operation is also carried out in the case of a rich or lean air-fuel ratio caused by a malfunction of the carburetor or during a driving of a car at a high altitude. If the control circuit generates the signal for the medium air-fuel ratio in such a condition, the actual air-fuel ratio of the mixture diverges to an extremely rich or lean value, which results in malfunctioning or stopping of the engine.